Oil reservoir permeability control

ABSTRACT

The permeability of a subterranean oil-bearing formation is controlled by the injection of an aqueous solution of a cross-linked block copolymer containing polar and non-polar segments, with the polar segments generally making up as least 50 percent, usually 60 to 99 percent, of the copolymer. The polar segments are derived from an alkylene oxide and the non-polar segments from styrene or an alkyl styrene either by itself or with a diene. The copolymers are cross-linked with an amino resin or a combination of a phenolic and a water-dispersible aldehyde component. The resulting copolymer gels are stable at low pH conditions, such as those of CO 2  flood conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of copending application Ser. No. 031,736, filed onMar. 30, 1987, now U.S. Pat. No. 4,776,398, which wascontinuation-in-part of copending applications, Ser. No. 696,952, filedon Jan. 31, 1985, now U.S. Pat. No. 4,653,585, Ser. No. 917,324, filedon Oct. 9, 1986, now U.S. Pat. No. 4,834,180, and Ser. No. 922,916,filed on Oct. 24, 1986, now U.S. Pat. No. 4,716,966, the entire contentsof all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the control of permeability in subterraneanoil-bearing formations.

2. Discussion of Related Art

In the production of oil from subterranean oil reservoirs by variousflooding techniques, especially waterflooding, it has become a commonexpedient to add various polymeric thickening agents to the water inorder to decrease its mobility to a point where it approaches that ofthe crude oil which is to be displaced so as to improve the displacementof the oil from the reservoir. Use of polymers for this purpose is oftenstated to be for mobility control.

Another problem which arises in the various flooding processes is thatdifferent strata or zones in the reservoir often possess differentpermeabilities so that displacing fluids enter the high permeability or"thief" zones in preference to zones of lower permeability wheresignificant quantities of oil may be left unless measures are taken toat least partially plug the high permeability zones to divert thedisplacing fluid into the low permeability zones. Mechanical isolationof the thief zones has been tried but vertical communication amongreservoir strata often renders such measures ineffective. Physicalplugging of the high permeability zones by cements and solid slurrieshas also been attempted with varying degrees of success but the mostserious drawback of this approach is the possibility of permanentlyclosing productive portions of the reservoir.

From these early experiences, the desirability of designing a viscousslug capable of sealing off the most permeable layers so that thefloodwater or other driving fluid, such as gas or steam, would bediverted to the underswept, tighter regions of the reservoir, becameevident. This led to the use of oil/water emulsions, gels and polymersfor controlling the permeability of the formations in a processfrequently called "profile control" or "flood conformance", a referenceto control of the vertical permeability profile of the reservoir.Profile control agents which have been proposed have included oil/wateremulsions, gels, e.g., lignosulfonate gels and polymers, with polymersbeing the most extensively applied in recent years.

Among the polymers so far examined for improving flood conformance arepolyacrylamides, polysaccharides, celluloses, furfural-alcohol andacrylic/epoxy resins, silicates and polyisocyanurates. A major part ofthis work has been conducted with the polyacrylamides, both in theirnormal, noncrosslinked form as well as in the form of metal complexes,as described, for example, in U.S. Pat. Nos. 4,009,755; 4,069,869 and4,413,680. In either form, the beneficial effects derived from thesepolyacrylamides seem to dissipate rapidly due to shear degradationduring injection and sensitivity to reservoir brines, low pH and hightemperature. To overcome these problems and to achieve deeperpenetration into the reservoir, dilute solutions of these polymers havesometimes been injected first and then complexed in situ.

Another group of polymeric thickeners which has received considerableattention for use in improving flooding are polysaccharides,particularly those produced by the action of bacteria of the genusXanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and3,383,307 disclose a process for mobility control by the use ofpolysaccharides. U.S. Pat. Nos. 3,741,307; 4,009,755; 4,069,869 disclosethe use of polysaccharides in the control of reservoir permeability.U.S. Pat. No. 4,413,680 describes the use of cross-linkedpolysaccharides for selective permeability control in oil reservoirs.

U.S. Pat. No. 3,908,760 describes a polymer waterflooding process inwhich a gelled, water-soluble Xanthomonas polysaccharide is injectedinto a stratified reservoir to form a slug, band or front of gelextending vertically across both high permeability and low permeabilitystrata. This patent also suggests the use of complexed polysaccharidesto block natural or man-made fractures in formations. The use ofpolyvalent metal ions for cross-linking polysaccharides is alsodisclosed in U.S. Pat. No. 3,810,882.

The use of various block copolymers for mobility control inwaterflooding operations is described in U.S. Pat. Nos. 4,110,232,4,120,801 and 4,222,881, but their use for permeability control has notbeen suggested.

Chung et al, in the aforementioned patent application Ser. No. 696,952,disclose the use of block copolymers, which may be cross-linked withpolyvalent metal ions, as permeability control agents in enhanced oilrecovery applications. However, polyvalent metal ions may not beeffective cross-linking agents under all conditions encountered in theenhanced oil recovery applications, e.g., in acidic conditions, such asthose encountered in carbon dioxide (CO₂) flooding operations.

There is therefore a continuing need for different types of polymer gelswhich are effective as permeability control agents in different types ofreservoirs under diverse reservoir conditions, including acidicconditions.

SUMMARY OF THE INVENTION

According to the present invention, the permeability of the reservoir iscontrolled by the selective placement within the reservoir of an aqueoussolution or dispersion of a cross-linked block copolymer of the AB orBAB type containing both polar and non-polar blocks. The copolymer iscross-linked either prior to the injection of the copolymer solution ordispersion, or in situ, by contacting the copolymer with at least onecross-linking agent which is an amino resin or a combination of aphenolic and a water-dispersible aldehyde component. The non-polarblocks of the polymer are derived from styrene or an alkylstyrene orhydrogenated diene-styrene units and the polar blocks are derived froman alkylene oxide. Generally, the polar blocks constitute at least about50% by weight of the polymer but in certain cases as little as 30% byweight is sufficient to ensure that the block copolymer will be swollenby water to form an aqueous gel having the desired properties.Generally, the polar units constitute about 60 to about 99, preferablyabout 60 to about 80, weight percent of the polymer. The solubility ofthe polymer may be enhanced by the presence of solubilizing groups, suchas sulfonate, in either the acid or salt form, i.e., as --SO₃ H or --SO₃M, where M is a metal, preferably an alkali metal, such as sodium, inwhich cases the proportion of the polar groups in the polymer may belower than it would otherwise need to be.

DETAILED DESCRIPTION OF THE INVENTION

We found that the amino resins or phenolic/water-dispersible aldehydecomponents are very effective cross-linking agents for the blockcopolymers used herein. These cross-linking agents effectively formrelatively stable gels even at pH conditions at which polyvalent metalsmay be ineffective as cross-linking agents or may form unstable gels.Thus, the cross-linking agents used herein form gels which are stableeven at acidic formation conditions, e.g., at pH of about 5.5 or less,commonly encountered in CO₂ flooding operations.

Amino resins used as cross-linking agents are known in the art and areprepared by reacting formaldehyde with urea or melamine, as described,for example, in Kirk-Othmer ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, ThirdEdition, Volume 2, John Wiley and Sons (1978), pages 440-467, the entirecontents of which are incorporated herein by reference. The resin mustbe soluble or dispersible in an aqueous medium. Non-limiting examples ofresins which can be used are melamine formaldehyde, urea-formaldehyde,ethylene and propylene urea formaldehyde, triazone, uran, and glyoxalresins. The amount of an amino resin required for polymer cross-linkingis about 0.1:1 to about 10:1 by weight of the polymer to the aminoresin. The most preferred amino resin is melamine formaldehyde resin,derived from a reaction of melamine and formaldehyde, which has a molarratio of melamine to formaldehyde of between about 1 to about 6. A ratioof between about 3 to about 6 is commonly used. Melamine formaldehyderesins are often fully or partially methylated to modify theirreactivity and solubility. The melamine formaldehyde resin can be acommercial product. Included among these melamine-formaldehyde(melamine) resins are the partially methylated resins and thehexamethoxymethyl resins (e.g., American Cyanamid's Cymel™ 373, Cymel370, Cymel 380 and Parez® resins).

If the cross-linking agent is the combination of a phenolic and awater-dispersible aldehyde component, any suitable water-dispersiblephenol or naphthol can be used as the phenolic component in the practiceof the invention. Suitable phenols include monohydroxy and polyhydroxynaphthols. Phenolic compounds suitable for use in the present inventioninclude phenol, catechol, resorcinol, phloroglucinol, pyrogallol,4,4'-diphenol, and 1,3-dihydroxynaphthalene. Other phenolic componentsthat can be used include at least one member of selected oxidizedphenolic materials of natural or synthetic origin, such as1,4-benzoquinone; hydroquinone or quinhydrone; as well as a natural ormodified tannin, such as quebracho or sulfomethylated quebrachopossessing a degree of sulfomethylation (DSM) up to about 50. (See U.S.Pat. No. 3,344,063, Col. 3, lines 15-32, which is incorporated herein byreference). The DSM of sulfomethylated quebracho (SMQ) is sometimesindicated by writing, for example, SMQ 50 for SMQ having a DSM of 50.Resorcinol and catechol are the preferred phenolic compounds for use inthe present invention for most permeability control applications.

Any suitable water-dispersible aldehyde can be used in the practice ofthe invention. Thus, under proper conditions of use, both aliphatic andaromatic monoaldehydes, and also dialdehydes, can be used. The aliphaticmonoaldehydes containing from one to about 10 carbon atoms per moleculeare preferred. Representative examples of such aldehydes includeformaldehyde, paraformaldehyde, acetaldehyde, propionaldehyde,butyraldehyde, isobutyraldehyde, valeraldehyde, heptaldehyde, decanal,and the like. Representative examples of dialdehydes include glyoxal,glutaraldehyde, terephthaldehyde, and the like. Various mixtures of saidaldehydes can also be used in the practice of the invention. The term"water-dispersible" is employed generically herein to include both thosealdehydes which are truly water-soluble and those aldehydes of limitedwater solubility but which are dispersible in water or other aqueousmedia to be effective gelling agents. Formaldehyde is the preferredaldehyde compound for use in the present invention.

Specific examples of suitable phenolic and water-dispersible aldehydecomponents are set forth in Swanson, U.S. Pat. No. 4,440,228, the entirecontents of which are incorporated herein by reference. The preferredcombinations of a phenolic and water-dispersible aldehyde components arephenol/formaldehyde and resorcinol/formaldehyde.

Relative amounts of phenolic and aldehyde components are also set forthin Swanson. These amounts are small but effective to cause the gelationof an aqueous dispersion of the copolymer and the cross-linking agent.The effective amount of aldehyde is about 0.002 to about 2, preferablyabout 0.1 to about 0.8 weight percent, based on the total weight of thecomposition comprising the water, the polymer and the cross-linkingagent.

Any suitable method can be employed for preparing the gelledcompositions of the invention. Thus, any suitable mixing technique ororder of addition of the components of said composition to each othercan be employed which will provide a composition having sufficientstability to degeneration by the heat of the formation (in which thecomposition is to be used) to permit good penetration of the compositioninto said formation. However, it is ordinarily preferred to firstdissolve or disperse the polymer in water before contacting the polymerwith the other components. The mixing order can vary with the type ofpolymer used. Some suitable mixing orders, with the components named inorder of mixing, include: water-polymer-amino resin; aminoresin-polymer-water; polymer-water-amino resin; water-polymer-phenoliccompound-aldehyde; water-phenolic compound-polymer-aldehyde; phenoliccompound-polymer-water aldehyde; and water-polymer-aldehyde-phenoliccompound. It is within the scope of the invention to moisten or slurrythe polymer with a small amount, e.g., about 1 to about 6 weightpercent, based on the weight of the polymer, of a small amount of a lowmolecular weight alcohol, e.g., C₁ to C₃ alcohols, as a dispersion aidprior to dispersing the polymer in water.

The gelled compositions of the invention can be prepared on the surfacein a suitable tank equipped with suitable mixing means, and then pumpeddown the well and into the formation employing conventional equipmentfor pumping gelled compositions. However, it is within the scope of theinvention to prepare said compositions while they are being pumped downthe well. For example, a solution of the polymer in water can beprepared in a tank adjacent the wellhead. Pumping of this solutionthrough a conduit to the wellhead can then be started. Then, downstreamfrom the tank, a suitable connection can be provided for introducing across-linking agent, e.g., an amino resin. As will be understood bythose skilled in the art, the rate of introduction of said componentsinto said conduit will depend upon the pumping rate of the polymersolution through said conduit. Any of the above-mentioned orders ofaddition can be employed in such an in situ technique. Mixing orificescan be provided in the conduit, if desired.

After the gelled composition is injected into the formation, the oilrecovery process is conducted in the usual manner, i.e., a displacingfluid, which is miscible or immiscible with the oil, is injected intothe formation. Suitable oil-miscible displacing fluids are carbondioxide (CO₂), carbon monoxide (CO), methane, ethane, propane, butane,natural gas, liquid petroleum gas and mixtures thereof. CO₂ is thepreferred oil-miscible displacing fluid. Suitable oil-immiscibledisplacing fluids are carbon dioxide, used under oil-immiscibleconditions, water or an aqueous fluid, nitrogen, ambient air, steam,flue gas, and mixtures thereof.

The gelled composition of the invention may also be used in a so-calledWAG (Water Alternating Gas) process, well known to those skilled in theart. In such a process, the injection of slugs of water is alternatedwith the injection of slugs of gas, such as CO₂. If a WAG process isused with the gelled compositions of the invention, the gelledcomposition or compositions are injected into the formation with one ormore water slugs.

After the miscible transition zone is established between the formationoil and the displacing fluid, a driving fluid may be injected throughthe injection well to displace the oil, the transition zone and thedisplacing fluid through the formation towards the production well fromwhich the oil is produced. The drive fluid is injected for a sufficienttime to effect the displacement of the formation oil to the productionwell until either all of the oil has been displaced from the formationor until the economical limit of the ratio of the driving fluid to theformation oil has been reached.

The drive fluid or driving fluid used in the process of the inventionmay be any drive fluid known to those skilled in the art, but preferablyit is a fluid selected from the group consisting of water, brine,methane, carbon dioxide, nitrogen, air, steam, separator gas, naturalgas, flue gas and mixtures thereof. The driving fluid may containadditives, such as a surfactant, to maintain efficient displacementthereof.

It is within the scope of the invention to precede the injection of thegelled composition into the well and out into the formation with apreflush of a suitable cooling fluid, e.g., water. Such fluids serve tocool the well tubing and formation and extend the useful operatingtemperature range of said compositions. The volume of said cooling fluidso injected can be any suitable volume sufficient to significantlydecrease the temperature of the formation being treated, and can varydepending upon the characteristics of the formation. For example,amounts up to 20,000 gallons or more can be used to obtain a temperaturedecrease on the order of 100° to 250° F.

The polymers which are used according to the present invention containpolar blocks and non-polar blocks and are of the AB or BAB type.Polymers of the BAB type include the following types: ##STR1## Blockcopolymers of this type in which the polar segments may be derived fromalkylene oxides or sulfides are described in U.S. Pat. Nos. 3,954,915and 3,867,295, the entire contents of which are incorporated herein byreference.

The block copolymers described in U.S. Pat. Nos. 3,954,915 and 3,867,295are intended for use in fuels and lubricants, e.g., as detergents anddispersants. As such, they were required to be soluble or dispersible inhydrocarbon and non-hydrocarbon oils, fuels and lubricants. To ensurethis, the amount of the polar block was kept below 50% of the weight ofthe polymer, preferably 5 to 25%. By contrast, the block copolymers usedin the present invention are to be soluble in water, at least to anextent that they will be swollen by the water or form micelles that willthicken the water to the extent necessary for effective permeabilitycontrol. Regardless of the actual solubility of the polymer in thewater, polymers of this kind which are either soluble in water or whichare swollen by water to form pumpable gels are referred to in thisapplication as being soluble in water and their mixtures with water arereferred to as being aqueous solutions even if the polymers formmicellar dispersions rather than true solutions.

In order to confer the requisite water solubility, the amount of thepolar units, which in the present case are derived from alkylene oxides,will generally be above 30% by weight of the polymer and more usuallyabove about 50% by weight although, if solubility-enhancing groups, suchas sulfonate, are present in the polymer, the proportion of polar unitsmay be somewhat lower. Normally, however, the proportion of polar unitswill be about 60 to about 80% by weight of the polymer, although up to99% by weight of these units may be present in the copolymer.

The materials used in the preparation of the polymers, e.g., alkylstyrene or mixtures of diene and styrene for the non-polar block,alkylene oxide for the polar block, solvents, anionic initiators,capping agents, etc. and the methods of preparation are those which aredescribed in U.S. Pat. Nos. 3,954,915 and 3,867,295 to which referenceis made for details of such materials and methods, except, as mentionedabove, the proportion of the polar and non-polar blocks will beappropriately modified to confer the desired solubility properties.Further, because solubility in organic liquids is not desired, thenon-polar blocks may be derived from styrene itself as well asalkylstyrenes and diene-styrene mixtures. In such cases, styrene will beused in the same way in the polymer preparation as the describedalkylstyrenes, with appropriate adjustment in weight amounts for thelower molecular weight.

Thus, in summary, the non-polar block A will be formed by anionicpolymerization of styrene or an alkylstyrene optionally with a diene.The diene-derived copolymers may be hydrogenated prior to the additionof the polar component of block B. The preferred polar components forblock B are alkylene oxides, e.g. ethylene oxide. Termination andhydrogenation of the diene units may be carried out as described in U.S.Pat. Nos. 3,954,915 and 3,867,295.

The desired molecular weights in the blocks and the final copolymer willgenerally be as previously described, but the weight of the polar blockwill generally be greater in order to confer the desired solubility inwater. Thus, the polar block will generally have at least 200 andpreferably at least 1000 alkylene oxide units, with a molecular weightof at least 50,000, typically 50,000 to 300,000 for the entire polymer.

In order to improve the solubility of the polymer in water, solubilityenhancing groups, such as sulfonate --SO₃ H or --SO₃ M, may be added bytreatment of the polymer with suitable reagents, e.g., by treating withsulfur trioxide the aromatic rings may be sulfonated and the acidsulfonic groups then neutralized with alkali, e.g., NaOH, KOH or NH₄ OH.However, care should be taken not to use forcing conditions which wouldbreak up the polymer. The copolymers containing diene, e.g., butadienesegments in the non-polar block may be sulfonated on the diene units bythe method described in U.S. Pat. No. 4,120,801 to which reference ismade for a description of the method, which employs a liquid sulfurtrioxide complex to sulfonate residual double bonds of the diene units.Alternatively, appropriately substituted styrenes or alkyl styrenes maybe used as the monomers in the production of the original blockcopolymer.

If reactive groups, especially sulfonate, are present in the copolymer,they may be converted to other solubilizing groups by conventionalchemical reactions. For example, sulfonate groups may be hydrolyzed tohydroxyl groups, as described in U.S. Pat. No. 4,120,801, orchloromethyl groups on the styrene unit may be quaternized by reactionwith tertiary amines to form a quaternary nitrogen group. The reactionof copolymers with styrene units to form chloromethylated copolymerswhich are then quaternized by reaction with tertiary amines is describedin U.S. Pat. No. 4,110,232, to which reference is made for details ofthese reactions.

The block copolymers are dissolved in water, at concentrations of about1,000 to about 20,000 ppm, preferably about 2,000 to about 5,000 ppm, toprovide the desired viscosity. The solution is then injected into theformation where it selectively blocks the more highly permeable regions,to control the subsequent flooding operation which may be carried out ina conventional manner. Injection of the solution into the formation maybe carried out in a conventional manner using an injection well whichextends from the surface of the earth into the formation, e.g. asdescribed in U.S. Pat. Nos. 4,078,607, 3,305,016, 4,076,074, 4,009,755and 4,069,869, to which reference is made for the descriptions oftypical procedures which can be used herein. Briefly, the thickenedaqueous liquid is injected into the formation through the injection welland in the formation it enters the more highly permeable stratum orstrata in preference to the less permeable regions because of itsviscosity. Once in place in the more highly permeable regions, the gelcontrols subsequent flooding operations by diverting the flood liquid,such as water or CO₂, to the less permeable or "tight" zones, increasingrecovery from these zones. The amount of the viscous solution which isinjected into the reservoir will generally be from 10% to 100% of thepore volume of the high permeability stratum or strata.

Because the compositions of the block copolymers may be readily varied,e.g., by changing the ratio of the polar to the non-polar blocks, theviscosifying effects of the polymers may also be varied. The blockpolymers therefore offer the possibility of formulating polymersaccording to specific reservoir conditions.

Because the alkylene oxide block copolymers possess a viscosifyingeffect in aqueous solutions, they may also be used for mobility controlpurposes in waterflooding operations. In carrying out waterflooding inthis way, at least a portion of the water injected into the oil-bearingformation through the injection well contains the block copolymer in anamount which is sufficient to thicken the water and increase itsviscosity to a point where it is closer to that of the oil, so as toincrease the efficiency of the displacement of the oil from theformation. Normally, the amount of the copolymer should be sufficient toachieve a mobility ratio equal to or less than 1 for the reservoir oilto the injected water, as described in U.S. Pat. No. 3,025,237, to whichreference is made. In many cases, the relative permeabilities of thereservoir to oil and water are discounted in arriving at the mobilityratio so that the desired viscosity of the thickened water will be atleast that of the reservoir oil, typically in the range of 1 to 4 timesthat of the reservoir oil. Continued injection of the water drives thedisplaced oil through the formation to the production well from which itis recovered. In order to reduce the cost of the flood, it may bepreferable to include the copolymer in only the initial portion of theflooding water; the concentration of the polymer may be decreasedgradually or stepwise after the initial portion and the portion which isinjected last may be free of the copolymer entirely. In this manner, aprogressive decrease in the viscosity of the flood water is achieved.

It will be apparent to those skilled in the art that the specificembodiments discussed above can be successfully repeated withingredients equivalent to those generically or specifically set forthabove and under variable process conditions.

From the foregoing specification, one skilled in the art can readilyascertain the essential features of this invention and without departingfrom the spirit and scope thereof can adapt it to various diverseapplications.

We claim:
 1. A cross-linked copolymer obtained by cross-linking a blockcopolymer having at least one polar and at least one non-polar segmentwith a cross-linking agent which is an amino resin or a combination of aphenolic component and a water-dispersible aldehyde component.
 2. Across-linked copolymer of claim 1 wherein the amino resin is produced byreacting formaldehyde with urea or melamine.
 3. A cross-linked copolymerof claim 1 wherein the cross-linking agent is phenol/formaldehyde orresorcinol/formaldehyde.
 4. A cross-linked copolymer of claim 3 whereinthe polar segment is derived from a polymerized alkylene oxide and thenon-polar segment from styrene or an alkylstyrene.
 5. A cross-linkedpolymer of claim 4 wherein the polar segment constitutes at least about50 weight percent of the copolymer.
 6. A cross-linked polymer of claim 5wherein the polar segment constitutes from about 60 to about 99 weightpercent of the copolymer.
 7. A cross-linked polymer of claim 6 whereinthe polar segment constitutes from about 60 to about 80 weight percentof the copolymer.
 8. A cross-linked polymer of claim 7 wherein thenon-polar segment comprises copolymerized styrene and diene units.
 9. Across-linked copolymer of claim 8 wherein the non-polar segmentcomprises polymerized alkylstyrene units.
 10. A cross-linked copolymerof claim 8 wherein the non-polar segment comprises polymerized styreneunits.
 11. A cross-linked copolymer of claim 10 wherein the copolymerhas solubilizing functional groups on the non-polar segment.
 12. Across-linked copolymer of claim 9 wherein the copolymer has solubilizingfunctional groups on the non-polar segment.
 13. A cross-linked copolymerof claim 11 wherein the solubilizing functional groups are sulfonategroups.
 14. A cross-linked copolymer of claim 12 wherein thesolubilizing functional groups are sulfonate groups.
 15. A cross-linkedcopolymer of claim 14 wherein the copolymer comprises at least 1000alkylene oxide units in the polar block.
 16. A cross-linked copolymer ofclaim 13 wherein the copolymer comprises at least 1000 alkylene oxideunits in the polar block.
 17. A cross-linked copolymer of claim 16wherein the copolymer has a molecular weight of from 50,000 to 300,000.18. A cross-linked copolymer of claim 15 wherein the copolymer has amolecular weight of from 50,000 to 300,000.
 19. A cross-linked copolymerof claim 2 wherein the polar segment constitutes at least about 50weight percent of the copolymer.
 20. A cross-linked copolymer of claim19 wherein the polar segment constitutes from about 60 to about 99weight percent of the copolymer.
 21. A cross-linked copolymer of claim20 wherein the polar segment constitutes from about 60 to about 80weight percent of the copolymer.
 22. A cross-linked copolymer of claim21 wherein the non-polar segment comprises copolymerized styrene anddiene units.
 23. A cross-linked copolymer of claim 22 wherein thenon-polar segment comprises polymerized alkylstyrene units.
 24. Across-linked copolymer of claim 22 wherein the non-polar segmentcomprises polymerized styrene units.
 25. A cross-linked copolymer ofclaim 24 wherein the copolymer has solubilizing functional groups on thenon-polar segment.
 26. A cross-linked copolymer of claim 23 wherein thecopolymer has solubilizing functional groups on the non-polar segment.27. A cross-linked copolymer of claim 25 wherein the solubilizingfunctional groups are sulfonate groups.
 28. A cross-linked copolymer ofclaim 26 wherein the solubilizing functional groups are sulfonategroups.
 29. A cross-linked copolymer of claim 28 wherein the copolymercomprises at least 1000 alkylene oxide units in the polar block.
 30. Across-linked copolymer of claim 25 wherein the copolymer comprises atleast 1000 alkylene oxide units in the polar block.
 31. A cross-linkedcopolymer of claim 30 wherein the copolymer has a molecular weight offrom 50,000 to 300,000.
 32. A cross-linked copolymer of claim 31 whereinthe copolymer has a molecular weight of from 50,000 to 300,000.